The invention relates to a rotor of a turbogenerator having direct gas cooling.
In generators which are operated according to the pressure-cooling principle, a main fan permits a swirled inflow of the cooling air into the overhang space of the pole zone of the rotor. That is, the residual swirl of the air at the outlet of the main fan ensures that the air rotates virtually without slip in front of the pole zone of the rotor. The inflow of cooling air into the mainly axial cooling passages, provided for this purpose, of the rotor and the rotor winding is therefore unproblematic in the case of the pressure-cooling principle.
In order to additionally increase the coolant flow rate in the rotor, it is proposed in DD 120 981, in the case of generators cooled by the pressure-cooling principle, to intensify the cooling of the rotor and its winding by means of an additional moving-blade cascade under the rotor cap. This moving-blade cascade is able to further reduce the so-called shock losses, which remain despite a residual swirl of the cooling air, when the cooling air enters the essentially axially running rotor cooling passages, so that the cooling of the rotor is optimized and the total losses are reduced.
In contrast, the main fans of generators working by the suction-cooling principle direct the cooling air first of all to a cooler, in the course of which the residual swirl of the cooling-air flow is rendered turbulent. In general, suction cooling, compared with pressure cooling, offers the advantage that the air leaving the coolers can be fed directly to the cooling passages in the entire generator and the temperature increase caused by the machine fan is eliminated. In this way, however, cooling air is fed to the rotor without the requisite swirl. The rotor must accelerate the cooling air first of all to peripheral velocity before it can enter the cooling passages. The rotor must therefore perform all the work in order to overcome the shock losses already mentioned. In the process, separation of the cooling-gas flow may occur, and the incident flow to the inlets of the rotor conductor elements may be incorrect. The consequence is then a greatly restricted cooling-air distribution with a corresponding risk to the entire rotor winding.
Although the use of a moving-blade cascade according to DD 120 981 is able to reduce this problem in the rotor cooling, which occurs in particular during suction cooling, it is unable to meet today""s requirements for the cooling of rotors in limit-rating machines. This is because it is necessary here to deflect the cooling-gas flow by up to 80xc2x0 during axial deflection, a factor which leads to the separation of the flow boundary layer at the blade wall in the case of a blade row proposed according to the prior art.
Accordingly, one object of the invention is to provide a novel rotor of a turbogenerator having direct gas cooling of the generic type, which rotor is preferably operated under suction cooling and can be cooled in an optimum manner.
The advantages of the invention may be seen, inter alia, in the fact that, due to a two-stage flow cascade, a desired pressure increase of the coolant flow is forced on the one hand in a first stage, and the requisite deflection of the coolant flow is carried out on the other hand in a second stage. Only such a functional separation between pressure increase and deflection of the coolant flow ensures an optimum incident flow, provided with minimum shock losses, to the cooling passages in the rotor body and in the rotor winding when applying the suction-cooling principle.
In an especially preferred embodiment of the invention, provision is made for the first stage of the flow cascade having pressure-generating properties to face the main fan of the electrical machine, and for the second stage of the flow cascade having deflecting properties to face the rotor winding overhang. Here, the flow cascades are separated from one another in axial direction; that is, they do not overlap one another in axial direction.
The moving blades of the stages of the moving-blade flow cascades have a curvature, whereas in the portion of the blades that are near to each other in the axial direction, the curvature of the blades of one stage of the moving blade flow is the same as the curvature of the blades of the adjacent stage of the moving-blade flow cascade in that portion. This arrangement prevents stalling of the cooling gas passing through the stages of the moving-blade flow cascades and thus to improve the rotor cooling.
A first spacing in a circumferential direction between adjacent moving blades of a stage of the moving-blade flow cascade is clearly greater than a second spacing in a circumferential direction between the blades of neighboring stages of the moving-blade flow cascade. Preferably the ratio of the second spacing to the first spacing is between 0.05 and 0.3.
An especially advantageous effect of the improved guidance of the coolant flow and thus of the cooling of the rotor appears if the walls between the inner margin of the rotor cap plate and the rotor shaft, which walls limit the coolant flow, have a contour converging conically toward the rotor winding overhang.